An imaging optical unit for EUV projection lithography is known from US 2007/0 223 112 A1, U.S. Pat. No. 6,781,671, US 2010/0 149 509 A1, WO 2012/137 699 A1, US 2012/0069314 A1, EP 1 768 172 B1, WO 2004/046 771 A1 and U.S. Pat. No. 7,751,127. The configuration of a system stop for an imaging optical unit is known from U.S. Pat. No. 6,445,510 B1.